First goal vital as Man United welcome Chelsea
United hold the whip hand following Wayne Rooney's goal in their 1-0 quarter-final first-leg win at Stamford Bridge and in most circumstances in the Champions League and for United, that would be that.
Sir Alex Ferguson's team have lost one of their last 30 matches in the competition at Old Trafford and none in the last six years. They have progressed in all 16 European ties where they won the first leg away from home.
This season they have conceded only two goals in nine Champions League games while in the Premier League they have won 15 and drawn one of their home games.
Since the Champions league began only two sides have recovered from a first-leg home defeat to win a knockout tie - Ajax Amsterdam against Panathinaikos in 1996 and Inter Milan, who recovered from a 1-0 defeat by Bayern Munich with a 3-2 win in Germany last month.
Yet the particular nature of this all-English tie means that such statistics are undermined in a game between two teams who know each other so well.
Although Chelsea have won only one of five away games in all-English Champions League ties, they won at Old Trafford in the Premier League a year ago and know that they need only one goal on Tuesday to swing the advantage their way.
Only two of the teams' last 12 competitive games have been decided by more than one goal so both sides know the Londoners are still very much in the tie.
"Of course Chelsea can do to us at Old Trafford what we've done there," said Ferguson. "We've only got a one-goal lead and we have to be very careful and give a top performance.
"It will be a tough game. The tie is not done."
Ferguson had the luxury of making eight changes from the first leg for Saturday's Premier League game against Fulham and those on duty came through unscathed - and barely out of breath - after a straightforward 2-0 win.
Antonio Valencia was on the scoresheet as he continues his rapid rehabilitation from a broken ankle to offer the manager plenty of attacking options after he decided to start with Nani on the bench at Stamford Bridge.
United looked sharper all over the pitch in that game but Chelsea could still easily have come away with a draw.
The Londoners looked off the pace again on Saturday as they stumbled past bottom club Wigan Athletic 1-0- a team they hammered 8-0 last May to clinch the title -- but have enough talent in the squad to score against anyone, anywhere.
"Obviously we know it will be difficult to win there but we have the confidence to do it, we did it last year," said manager Carlo Ancelotti.
"We are at a disadvantage now but we played well (in the first leg), we should have had a penalty, we hit the post."
Ancelotti's biggest headache is how to best use his trio of front men - Didier Drogba, Nicolas Anelka and Fernando Torres, who took his goal drought to 10 matches after coming off the bench against Wigan.
The Italian started with Drogba and Torres in the first leg in a 4-4-2 formation that seemed to squeeze the space from his midfield and could well turn to Anelka this time and revert to 4-3-3.
Florent Malouda, the scorer against Wigan, could also start while centre-back Alex and midfielder Yossi Benayoun offer further options after returning from injury.
Manchester United: 1-Edwin van der Sar; 21-Rafael da Silva, 15-Nemanja Vidic, 5-Rio Ferdinand, 3-Patrice Evra; 25-Antonio Valencia, 16-Michael Carrick, 11-Ryan Giggs, 13-Park Ji-sung; 14-Javier Hernandez, 10-Wayne Rooney.
Chelsea: 1-Petr Cech; 2-Branislav Ivanovic, 33-Alex, 26-John Terry, 3-Ashley Cole; 7-Ramires, 8-Frank Lampard, 5-Michael Essien, 15-Florent Malouda; 39-Nicolas Anelka, 9-Fernando Torres.
Referee: Olegario Benquerenca (Portugal)